It is common practice to control a shut-off valve in a fluid line by means of a monitor pilot in combination with a 3-way actuator pilot. For this purpose a control cylinder incorporated in the structure of the shut-off valve has a piston to open the shut-off valve in response to the pressure of control fluid and in the absence of the pressure from the control fluid the shut-off valve closes by spring pressure and/or line pressure. The 3-way actuator pilot normally places the control cylinder in communication with a source of pressurized control fluid to keep the shut-off valve open, but when the actuator pilot is vented through a pilot port it not only cuts off the control fluid from the control cylinder, but also releases control fluid from the control cylinder to cause the shut-off valve to close by means of spring pressure and/or line pressure. The monitor pilot senses changes in the pressure in the fluid line and controls the actuator pilot accordingly. For this purpose the monitor pilot controls a relief valve which is connected to the pilot port of the actuator pilot and which opens whenever a pressure change in the fluid line indicates that the shut-off valve should be closed.
The present invention is directed to improvements in such a control system.